It is conventional for a fiber web leaving a carder to be conveyed to the calendering cylinders for consolidating the web by means of a conveyor belt. Until now, the fiber web has been transferred from the conveyor belt to the two calendering cylinders by the web being taken up directly by the calendering cylinders, with the conveyor belt extending so as to be tangential to the two calendering cylinders.
On being transferred, and while in the intermediate zone between the conveyor belt and the calendering cylinders, the web is unsupported, and that is harmful to its cohesion. In addition, in the intermediate zone, the conveyor belt and the calendering cylinders generate air turbulence because they are in motion, and the greater the speed of the conveyor belt and of the calendering cylinders, the greater the turbulence, which gives rise to an increased risk of transverse creases forming in the web while it is being transferred.
In order to reduce the effects of air turbulence on the web, European patent application EP 0 155 656 has already proposed compressing the web prior to transferring it. In a particular variant embodiment described in that European patent application, use is made of a hollow perforated rotary cylinder which is positioned above the conveyor belt, upstream from the two calendering cylinders. When the web reaches the hollow cylinder, it is subjected to compression prior to being transferred, with the air initially contained in the fiber web escaping by passing through the perforated cylinder.
Such prior compression of the fiber web serves to attenuate the effects of the zone of turbulence, but it does not prevent the web being unsupported while it is being transferred.
In addition, in practice, it is necessary for the fiber web to be taken up directly at the outlet of the conveyor belt by the calendering cylinders while simultaneously being subjected to considerable stretching in the length direction of the fiber web, which stretching may be as great a 50% for linear speeds of the conveyor belt of the order of 100 meters per minute (m/min). Unfortunately, on leaving the carder and prior to being calendered, the fiber web has very little cohesion. Consequently, when it is stretched lengthwise, the cohesion of the web is reduced correspondingly. When the web is stretched too much, then a web is obtained that is of poor quality with respect to appearance, uniformity of weight, and isotropy of its mechanical properties. This drawback associated with web stretching is particularly critical with scrambled and/or condensed webs which have less longitudinal strength than do parallel webs.
The above-mentioned problems of the fiber web being unsupported and being stretched while it is being transferred put a limit on the speed at which the fiber web can be conveyed prior to being consolidated by the calendering cylinders. In practice, a web coming from a carder and not subject to intermediate consolidation treatment cannot be conveyed by a conveyor belt and taken up directly by calendering cylinders at a speed greater than 120 m/min.